The embodiments herein relate generally to subterranean formation operations and, more particularly, to forming proppant-free channels in propped vertically oriented fractures.
Subterranean wells (e.g., hydrocarbon producing wells) are often stimulated by hydraulic fracturing treatments. In hydraulic fracturing treatments, a treatment fluid is pumped into a portion of a subterranean formation at a rate and pressure such that the subterranean formation breaks down and one or more fractures are formed. Typically, particulate solids are then deposited in the fractures. These particulate solids, or “proppant particulates” or “proppant,” serve to prevent the fractures from fully closing once the hydraulic pressure is removed by forming a proppant pack. As used herein, the term “proppant pack” refers to a collection of proppant particulates in a fracture. By keeping the fracture from fully closing, the proppant particulates aid in forming conductive paths through which fluids may flow.
The degree of success of a fracturing operation depends, at least in part, upon fracture porosity and conductivity once the fracturing operation is stopped and production is begun. The porosity and conductivity of a proppant pack is thus related to the interconnected interstitial spaces between the abutting proppant particulates in the proppant pack, and, accordingly, closely related to the strength of the placed proppant (e.g., if the placed proppant crushes, then the pieces of broken proppant may plug the interstitial spaces) and the size and shape of the placed proppant (e.g., more spherical proppant particulates generally yield increased interstitial spaces between the particulates).
When fractures close on a proppant pack upon removal of hydraulic pressure, the fractures may crush or compact the proppant particulates, potentially forming non-permeable or low permeability masses within the fracture, rather than desirable high permeability masses. Such low permeability masses may choke the flow path of the fluids within the formation. Furthermore, the proppant particulates may become embedded in particularly soft formations, negatively impacting production.
One way to increase conductivity of proppant packs involves the placement of proppant aggregates comprised of multiple individual proppant particulates. The larger size of the proppant aggregates compared to individual proppant particulates allows a reduced volume of proppant to be placed into the fracture while maintaining the structural integrity required to keep the fracture from closing and crushing the proppant aggregates. Accordingly, the spaces between the proppant aggregates through which produced fluids flow may be larger than the interstitial spaces that would be present between individual proppant particulates. Typical proppant aggregates are formed by agglomerating proppant particulates using a resin or tackifying agent, for example, that may, or may not, remain tacky after the aggregates have formed.
Another method proposed to increase the conductivity of proppant packs is to pump a substantially solids free fluid intermittently between pumping proppant particulates. The solids free fluid forms spaces within the proppant pack by preventing individual proppant particulates from gathering particularly close to one another. These spaces, or “proppant-free channels,” form conductive channels through which produced fluids may flow. Such intermittent pumping may be deleterious to operational equipment, as it requires the constant turning on and off of the equipment. Additionally, the intermittent pumping may cause additives in either the solids free fluid or other treatment fluids to settle out during the constant pressure changes (i.e., as the pumping equipment is stopped and begun again) and/or deposition of the additives in undesired locations in the subterranean formation.